An electrodynamic transducer may be utilized as a loudspeaker or as a component in a loudspeaker system to transform electrical signals into acoustical signals. The basic designs and components of various types of electrodynamic transducers are well-known.
An electrodynamic transducer typically includes mechanical, electromechanical, and magnetic elements to effect the conversion of an electrical input into an acoustical output. For example, the transducer typically includes a frame, a magnetic motor assembly that provides a magnetic field across an air gap, a voice coil, a diaphragm having an outer perimeter and an apex, and a suspension system coupled between the outer perimeter of the diaphragm and the outer perimeter of the frame. The voice coil, supported by a former, is coupled to the apex of the diaphragm so that the electrical current that flows through the voice coil causes the voice coil to move in the air gap and also causes the diaphragm to move.
The motor assembly and voice coil cooperatively function as an electromagnetic transducer (also referred to as simply a transducer, loudspeaker, or driver). The motor assembly typically includes a magnet (typically a permanent magnet) and associated ferromagnetic components-such as pole pieces, plates, rings, and the like-arranged with cylindrical or annular symmetry about a central axis. However, any device that creates a static magnetic field may be used, including field coil motors with no permanent magnets. Moreover, other magnet arrangements may be used, such as square, race track or other asymmetric configurations.
Taking the annular configuration as a typical example, the motor assembly establishes a magnetic circuit in which most of the magnetic flux is directed into an annular (circular or ring-shaped) air gap (or “magnetic gap”), with the lines of magnetic flux having a significant radial component relative to the axis of symmetry. The voice coil typically is formed by an electrically conductive wire cylindrically wound for a number of turns around the lower portion of the voice coil former, while the upper part of the voice coil former is attached to the diaphragm. The coil former and the attached voice coil are inserted into the air gap of the magnetic assembly such that the voice coil is exposed to the static (fixed-polarity) magnetic field established by the magnetic motor assembly. The voice coil may be connected to an audio amplifier or other source of electrical signals that are to be converted into sound waves.
In a conventional construction, the diaphragm of the transducer (also called “a cone” because of its shape) is formed as a cone that is substantially greater in diameter than the voice coil. In this type of construction, the diaphragm includes a flexible or compliant material that is responsive to a vibrational input. The diaphragm is suspended by one or more supporting but compliant suspension members such that the flexible portion of the diaphragm is permitted to move. In common constructions, the suspension members may include an outer suspension member known as a “surround.” The surround is connected to the diaphragm's outer edge and extends outward from the diaphragm to connect the diaphragm to the frame. The supporting elements may also include an inner suspension known as “spider.” The spider is typically connected to the voice coil and extends from the voice coil to a lower portion of the frame; thus, connecting the voice coil to the frame. In this way, the diaphragm is mechanically referenced to the voice coil, typically by being connected directly to the former on which the voice coil is supported.
In operation, electrical signals are transmitted as an alternating current (AC) through the voice coil in a direction substantially perpendicular to the direction of the lines of magnetic flux produced by the magnets. The alternating current interacts with the constant magnetic field in the magnetic air gap. The interaction results in a Laplace force. This force is expressed as a product of the magnetic flux density, overall length of the voice coil's turns linked to the magnetic flux, and the value of the electrical current running through the voice coil. Due to the Laplace force acting on the coil wire positioned in the permanent magnetic field, the alternating current corresponding to electrical signals conveying audio signals actuates the voice coil to reciprocate back and forth in the air gap and, correspondingly, move the diaphragm to which the coil (or coil former) is attached. Accordingly, the reciprocating voice coil actuates the diaphragm to likewise reciprocate and, consequently, produce acoustic signals that propagate as sound waves through a suitable fluid medium such as air. Pressure differences in the fluid medium associated with these waves are interpreted by a listener as sound. The sound waves may be characterized by their instantaneous spectrum and level, and are a function of the characteristics of the electrical signals supplied to the voice coil.
Because the material of the voice coil has an electrical resistance, some of the electrical energy flowing through the voice coil is converted to heat energy instead of sound energy. The heat emitted from the voice coil may be transferred to other operative components of the loudspeaker, such as the magnetic assembly and coil former. The generation of resistive heat is disadvantageous for several reasons. First, the conversion of electrical energy to heat energy constitutes a loss in the efficiency of the transducer in performing its intended purpose—that of converting the electrical energy to mechanical energy utilized to produce acoustic signals. Second, excessive heat may damage the components of the loudspeaker and/or degrade the adhesives often employed to attach various components together, and may even cause the loudspeaker to cease functioning. For instance, the materials of certain components themselves, as well as adhesives and electrical interconnects (e.g., contacts, soldered interfaces) may melt, become fouled, or otherwise degraded.
As additional examples, the voice coil may become detached from the coil former and consequently fall out of proper position relative to other components of the driver, which adversely affects the proper electromagnetic coupling between the voice coil and the magnet assembly and the mechanical coupling between the voice coil and the diaphragm. Also, excessive heat will cause certain magnets to become demagnetized; for example, different grades of neodymium (Nd) magnets will demagnetize at temperatures between about 80° C. and 200° C. Thus, the generation of heat limits the power handling capacity and distortion-free sound volume of loudspeakers as well as their efficiency as electro-acoustical transducers. Such problems are exacerbated when one considers that electrical resistance through a voice coil increases with increasing temperature. That is, the hotter the wire of the voice coil becomes, the higher its electrical resistance becomes and the more heat it generates.
The most common form of a loudspeaker uses a single voice coil winding in a single magnetic gap. However, loudspeaker performance may be enhanced by using a multiple coil/multiple gap design.
A multi-coil transducer may include two or more separate windings axially spaced apart from each other to form two or more coils, although the same wire may be employed to form the coils. The multiple voice coils are usually electrically connected together either on the coil itself or on the outside of the loudspeaker so that the coils work together to move the diaphragm. As both coils provide forces for driving the diaphragm, the power output of the loudspeaker may be increased without significantly increasing size and mass. The most common implementation of the multiple coil loudspeaker uses two voice coils and two magnetic gaps, however, additional voice coils may serve other purposes besides driving the cone, such as limiting excessive excursion or providing a sense signal that indicates coil velocity or position or other functions.
Many multi-coil/multi-gap designs are able to produce more power output per transducer mass and dissipate more heat than conventional single-coil designs. For example, a dual-coil design provides more coil surface area compared with many single-coil configurations, and, thus, ostensibly is capable of dissipating a greater amount of heat at a greater rate of heat transfer. A dual-coil design that doubles the surface area and number of turns of the coil winding may increase (e.g., nearly double) the capacity of the coil to dissipate heat.
While the multiple coil/multiple gap construction has several advantages over single gap designs including higher power handling, reduced distortion, reduced inductance, and extended frequency response, there are at least three particular disadvantages with dual coil/dual gap speakers. First, insofar as a desired advantage of the dual-coil driver is its ability to operate at a greater power output, so operating the dual-coil transducer at the higher power output concomitantly causes the dual-coil transducer to generate more heat. Hence, the improved heat dissipation inherent in the dual-coil design may be offset by the greater generation of heat. There can be problems with overheated magnets due to the compact motor and the proximity of the magnets to the heat-generating voice coils. For example, as compared to single-coil transducers, adequate heat dissipation in many dual-coil transducers, and more generally multiple-coil transducers, continues to be a problem due to the longer thermal paths that must be traversed between the heat source (primarily the voice coil) and the ambient environment.
Second, the longer dual voice coil and motor structure add to the overall depth of the loudspeaker and this can limit the usability in applications with limited available space.
Third, the longer dual voice coil is cantilevered at the extreme back of the loudspeaker, far removed from the suspension elements. In this position the voice coil is prone to wobble or lash radially in the magnetic gap, possibly striking the magnet structure. An additional drawback of having a deeper profile requires more space inside the speaker enclosure.
Accordingly, a need therefore exists for a compact to multiple voice coil/multiple gap transducer construction providing increased power handling and means for rapidly removing significant amounts of heat from electrically conductive coil structures and magnetic structures during the operation of transducers and transducer-containing devices such as loudspeakers and the like.